Journal of the Japan Society of Engineering Geology

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In selecting and constructing sea area landfill sites, the existence of low-permeable clay layer with certain thickness and sealing capability over the seabed is investigated and confirmed, according to the regulation to guarantee safety for leakage of polluted water from the bottom of the site.
During landfill operation, however, certain risky condition may arise when solids of large size are dumped. They fall down through the water and collide to the clay layer with high settling speed, so that such impacts may cause deterioration of sealing capability of the clay layer.
We, therefore, made experiments for observing behavior of settling solids of different densities and shapes using a transparent water cylinder. In addition, using a real site, the settling speeds of dumped concrete blocks were measured with an underwater camera, while the impact forces of solids to the bottom were measured with an accelerometer. As the results, it was found that a group of large solids settles down rapidly, and some large particles partly penetrate into the bottom layer. The impact force and depth of penetration observed in the experiments mostly coincides with the estimated values by Hertz's theory.

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Landslide distribution mapping with chronological research is studied in the Wasabizawa Area, southeastern Akita Prefecture. Geomorphological features of landslides, i.e., dissection features of scarps, dissection degree of moving masses, and morphologic relationships to river channels are introduced into the classification of formation. Chronological evidences such as tephra and ¹⁴C ages from some representative moving masses are consistent with the geomorphological classification. The estimated primary moving ages of three masses are 35-40 ka, before 10-20 ka, and around 10 ka, while that of one is almost 1 ka. Geomorphological features of low-relief surfaces related to 200-250ka volcanism are also suggested as an indicator of stable body since Middle Pleistocene, of which inactivity is ascertained by the boring survey.

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